This generally tidies up and documents how we substitute options and
environment variables in get_simulator_cmd. It's now slightly laxer,
so it won't moan about a missing environment variable or option if
it's not actually used in the command.
This means we can wrap main() in a try/except block. That way, future
patches can change code that fails to throw an exception, rather than
having to do the ugly "write to stderr; exit 1" dance.
This also catches a global (cwd) which we were passing implicitly to
rtl_sim. Fortunately, we already have that value in _CORE_IBEX, so can
just use that.
We now put sys.path back after importing things and no longer import
'*' from anything (which means static analysis tools can now spot when
we've got a name wrong).
The process_regression_list function from riscv-dv is really chatty.
When we're just compiling the testbench, we don't actually need to
figure out what tests we're running, since we'll never run them
anyway. So don't call it.
